(1) Field of the Invention
The present invention relates to a process for manufacturing a steel having MnS finely and uniformly precipitated therein, with a deoxidation product as a nucleus, by specially a continuous casting method.
(2) Description of the Related Art
Requirements for the properties of materials of large constructions such as marine structures, ships and store tanks have become severe, and drastic improvements of toughness are desired. In general, in the austenite-to-ferrite transformation of steel, coarse ferrite is precipitated from the grain boundary of austenite and the texture becomes coarse. Furthermore, when a steel material is subjected to automatic welding such as submerged arc welding, electro-gas welding or electroslag welding, the texture of the welding heat-affected zone (hereinafter referred to "HAZ") is further coarsened. It is known that the toughness of the steel is reduced as the texture is coarsened, and various processes for making the texture fine have been proposed, for improving the toughness.
For example, Japanese Unexamined Patent Application No. 61-238940 discloses a process for making the texture fine, in which the intragranular ferrite plates (called "IFP" hereinafter) are formed in austenite grains by using inclusions dispersed in steel as transformation nuclei, and "Tetsu-to-Hagan", Vol. 65, No. 8 (1979), page 1232 teaches a method for improving the HAZ toughness of a high strength steel of 50 kg/mm.sup.2 upon a large heat input welding by finely dispersing and precipitating TiN. However, this method is defective in that, in the vicinity of the bonded portion, coarsening by a re-dissolution of TiN and degradation of the HAZ toughness by an increase of dissolved N cannot be avoided.
Several inclusions such as MnS, TiN, REM, Ca, TiO and Ti.sub.2 O.sub.3 have been found effective as the transformation nucleus of IFP, but since MnS is generally precipitated in a micro-segregated portion having a high hardenability, it is difficult for MnS to act as a transformation nucleus of IFP.
In "Tetsu-to-Hagan", 1987, S197 and JSPS, 19th-Com. Rept. 10835 (1987), the present inventors clarified that deoxidation is carried out with Al, Ti, Zr or the like and fine MnS is precipitated by using the deoxidation product as the nucleus, but it is necessary to increase the number of MnS precipitates as the transformation nuclei of IFP in steel.
Accordingly, to make the texture fine by utilizing IFP, i.e., to drastically increase the toughness, a method must be established in which (1) an inclusion (for example, MnS) acting as the transformation nucleus of IFP is made uniform and ultrafine, and (2) an inclusion acting as the transformation nucleus of IFP even upon large heat input welding, and having an excellent high-temperature stability, is uniformly and ultrafinely dispersed.